This invention relates to the improvement of a speed change control device for a continuously variable transmission used in a vehicle etc.
One type of speed change control device known in the art which is widely used employs oil pressure, for example as disclosed in Tokkai Hei 11-2302.
In this control device, a speed change control valve of a toroidal continuously variable transmission takes the form of a spool. The spool is connected to a midway point on a speed change link which is free to swing. One end of the speed change link is connected to an actuator, and the other end is connected with a mechanical feedback mechanism which feeds back the gyration angle (rotation angle) of a power roller.
The differential pressure of two oil chambers of a hydraulic cylinder is adjusted according to the displacement of the spool driven by the actuator. A trunnion which supports a power roller causes the power roller to gyrate when the trunnion is driven in an axial direction, and provides support for the transmission torque applied to the power roller.
In such a toroidal continuously variable transmission, a high flowrate of lubricating oil is needed for rolling surfaces of the power roller and input and output disks, lubrication of bearings and cooling. When the oil temperature is high, the flowrate of lubricating oil must be even larger to suppress excessive temperature rise of the power rollers, so the discharge flowrate of an oil pump must be set larger than in the case of an automatic transmission using a planetary gear set.
However, in the neutral position of a speed change control valve as in the aforesaid prior art speed change control device for a continuously variable transmission, if the relation of the ports and lands of the spool is set so that they underlap in order to satisfy the dual conditions of speed change control stability and response, each port will be open a fraction even in the neutral position when speed change is not performed. As a result, oil is discharged from the supply ports to the drain ports. The oil used for speed change control is the same as the lubricating oil used for lubrication, so the discharge flowrate of the oil pump must be set even higher. To ensure that there is both sufficient oil flowrate to provide oil stability of speed change control and sufficient oil flowrate for lubricating oil, the load on the engine driving the oil pump increases. In particular, to ensure sufficient discharge flowrate
when the engine rotation speed is low, the discharge flowrate specific to the oil pump must be increased, so the pump has to be made larger and fuel-consumption performance may be impaired.
It is therefore an object of this invention, which was conceived in, view of the above problems, to ensure stability of speed change control even when the discharge flowrate is reduced in the neutral position of a speed change control valve in an effort to improve fuel-consumption performance.
In order to achieve above object, this invention provides a speed change control device for a speed change control device for a continuously variable transmission comprising; a trunnion which supports power rollers gripped between input/output disks free to rotate, and is able to rotate around an axis and displace in axial direction,a hydraulic cylinder which drives the trunnion in an axial direction, and a speed change control valve which controls an oil pressure supplied to this hydraulic cylinder, wherein the speed change control valve houses a spool free to displace, the spool comprising; a spool which is housed in the speed change control valve and provides a first land which can face a supply port supplying a supply pressure and second lands which can respectively face first and second drain ports formed on either side of the supply port, first output port is provided for allowing first oil chamber provided in the hydraulic cylinder to selectively communicate with the supply port or the first drain port according to the displacement of the spool, second output port is provided for allowing second oil chamber provided in the hydraulic cylinder to selectively communicate with the supply port or the second drain port according to the displacement of the spool, the first land faces the supply port and the second lands face the first and second drain ports in the neutral position of the spool, and the first land closes the supply port and the second lands are formed such that the first drain port communicate with the first output port which is formed between the first drain port and the supply port, the second drain port communicates with the second output port which is formed between the second drain port and the supply port in the neutral position of the spool.